1. Field of the Invention
The present invention relates to a narrow band high sensitivity photo-detector, which belongs to the technical fields of electron technology and photon technology and is used for a high resolution inverse photoemission spectroscope.
2. Related Art Statement
A band pass filter type photo-detector for inverse photoemission spectroscopy is formed of a simple combination of low-cut and high-cut filters. In a recently developed photo-detector having excellent properties, the low-cut filter is a photo-electron multiplier provided with Cu-BeO having a KCl thin film deposited on a surface thereof for the first dynode, while the high-cut filter is a SrF.sub.2 monocrystal window having a sensitivity property of central energy at 9.40 eV and a half value width of 0.47 eV, as shown in FIG. 1.
This band pass filter type photo-detector is characterized by a structure that is simple and an operational stability that is high. However, the obtainable resolution thereof is no more than 0.47 eV at most.
Photoemission spectroscopy and inverse photoemission spectroscopy are means for directly observing occupied and unoccupied electronic states of energy in a material, respectively, and hence have a complementary relation with each other. The presently standardized resolution of photoemission spectroscopy is on the order of 0.3 eV. The investigation of occupied and unoccupied electronic states of energy with a precision of the same order by the employment of photoemission and inverse photoemission spectroscopies respectively is extremely desirable for the research of materials. It is necessary for the realization thereof to narrow the bandwidth of the band pass filter type photo-detector into the order of 0.3 eV.
In general, when an attempt is made to obtain higher resolution, the detection sensitivity is lowered in correspondence therewith. This is also true of the band pass filter type photo-detector. If the surface of the material to be measured is unstable in vacuum, an experiment wherein signals are accumulated for a long time is impractical. In this case, it is required to raise the detection sensitivity at the sacrifice of resolution. However, it is not possible to vary the width of the passband of the conventional band pass filter type photo-detector during the experiment. Accordingly, it is required for achieving high resolution to provide a measuring apparatus comprising two kinds of detectors, a high resolution detector, even if the detection sensitivity is somewhat low, and a high sensitivity detector.
Photoemission spectroscopy and inverse photoemission spectroscopy present information relating to occupied and unoccupied electronic states respectively, so that the whole aspect of the electronic states of a material cannot be clarified until both kinds of information are combined. The resolution of conventional photoemission spectroscopy is on the order of about 0.3 eV, while the resolution of conventional inverse photoemission spectroscopy is on the order of about 0.5 eV. Accordingly, it is necessary for comparing and investigating respective data from both of these spectroscopies to provide comparable resolutions thereto. Inverse photoemission spectroscopy is a method for clarifying the unoccupied electronic state of a material by applying a monochromatized electron to a sample, taking notice of a certain single energy component of light emitted from the surface of the sample, and by observing the intensity of the emitted light as a function of the energy of the noticed electron beam. The energy width of the electron beam is defined by the expansion of a thermo-electron in the electron source, which is about 0.25 eV. On the other hand, the width of the pass band of the band pass filter type photo-detector is conventionally about 0.47 eV and has been improved to 0.35 eV by recent technical development. However, to obtain resolution comparable with that of photoemission spectroscopy, it is necessary to narrow the width of the passband of the photo-detector to the order of the electron beam width of 0.25 eV.
In general, when the resolution is raised, the detection sensitivity is lowered. This is also true of the band pass filter type photo-detector for inverse photoemission spectroscopy, so that, when the width of the detector is narrowed to half value, the detection sensitivity is lowered and hence a longer time is required for the measurement. On the other hand, when the width of the detector is widened to half value, the sensitivity is raised, and thereby the measurement is carried out in a shorter time. In general, whether either the resolution or the sensitivity is preferential is decided by the limitation of the time required for the measurement and whether the material to be measured is stable or not in super high vacuum wherein the inverse photoemission spectroscopy is effected.
The resolution of inverse photoemission spectroscope is primarily defined by the bandwidth of the band pass filter type of photo-detector.
Two types of conventional photo-detectors are used in practice, that is, the photo-electron multiplier type and the Geiger-Mueller tube type. With respect to stability and affinity to super-high vacuum, a photo-detector of the photo-electron multiplier type is irresistively advantageous, and hence is adopted by most of the research groups in the world. The photo-detector according to the present invention also belongs to this type. However, the conventional photo-detector is inferior with respect to resolution in spite of its superiority in stability and super-high vacuum performance. The bandwidth of a recently used photo-detector of this type is a minimum of about 0.47 eV.